As a technique for allowing an observer to recognize a stereoscopic image, generally, a stereoscopic image display method based on binocular parallax using a difference between positions of the left eye and the right eye is used. This method is an application of the principle of stereogram that a three-dimensional stereoscopic image is recognized based on a difference between viewing methods in the brain by allowing the left eye and the right eye to visually recognize mutually-different two-dimensional images. As methods for displaying a stereoscopic image, there are a system using glasses and a naked-eye system not using glasses. As the naked-eye system, there are a two-eye system, a multiple eye system, and the like in accordance with the number of observer's viewpoints.
In order to represent a stereoscopic image based on the naked-eye system by using a two-dimensional display such as a general flat panel display, pixels displaying a left-eye image and a right-eye image are arranged in a two-dimensional display. An optical unit such as a lenticular lens having a cylindrical lens arranged therein or a parallax barrier having a slit-shaped light shielding pattern arranged therein is disposed between the two-dimensional display and an observer. Then, a method is used in which the left-eye image and the right-eye image on the screen are spatially separated so as to be visually recognized respectively by the left and right eyes.
In the technology described above, while an image is stereoscopically displayed on the screen of a two-dimensional display, a technology for displaying an image in which an object is represented as if it floats in the air has also been developed. For example, a method of stereoscopically displaying a three-dimensional image in accordance with a volume scanning method has been proposed in which a two-dimensional display is arranged to be inclined with respect to the optical axis of an imaging optical system by using the imaging optical system such as a convex lens or a concave mirror, the two-dimensional image inclining with respect to the optical axis is moved through mirror scanning, and a three dimensional image is formed by displaying a cross-section image of a display object on the two-dimensional display in synchronization with the moving two-dimensional image.
In Japanese Patent Application Laid-Open No. 2013-080227, a volume scanning-type three-dimensional air image display apparatus is disclosed which includes a real mirror image forming optical system that can form a real image of a projecting object at plane symmetrical positions with respect to one geometric plane that is a symmetrical plane as mirror images, a display that is arranged on the lower face side of the symmetrical plane and includes a display face displaying an image as the projecting object, and a drive unit that operates the display to perform movement including a component of a vertical direction with respect to the display face and, by changing an image displayed on the display face in synchronization with the operation of the display performed by the drive unit, forms the image as a stereoscopic image in a space of the upper face side of the symmetrical plane.